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PubMed1993

Computer simulation of counterlungs.

We have developed a computer model of chest-mounted counterlungs, which accounts for counterlung shape, effective volume, and pressure centroid. The model has been validated and the principles are applicable to other counterlung systems. The highly non-linear and discontinuous behavior of a counterlung is predicted by use of a sophisticated numerical integration method that computes variables such as pressure and volume in the time domain. Three separate stiffness (reciprocal of compliance) terms have been used which contribute to the diver's work of breathing: material elastic stiffness, "gas" stiffness, and "hydrostatic" stiffness. The model provides a significant advance in the understanding of counterlung behavior, allowing the performance of practical equipment to be predicted.

Computer Simulation↗

PubMed1990

Restoring unassisted natural gait to paraplegics via functional neuromuscular stimulation: a computer simulation study.

Functional neuromuscular stimulation (FNS) of paralyzed muscles has enabled spinal-cord-injured patients to regain a semblance of lower-extremity control, for example to ambulate while relying heavily on the use of walkers. Given the limitations of FNS, specifically low muscle strengths, high rates of fatigue, and a limited ability to modulate muscle excitations, it remains unclear, however, whether FNS can be developed as a practical means to control the lower extremity musculature to restore aesthetic, unsupported gait to paraplegics. A computer simulation of FNS-assisted bipedal gait shows that it is difficult, but possible to attain undisturbed, level gait at normal speeds provided the electrically-stimulated ankle plantarflexors exhibit either near-normal strengths or are augmented by an orthosis, and at least seven muscle-groups in each leg are stimulated. A combination of dynamic programming and an open-loop, trial-and-error adjustment process was used to find a suboptimal set of discretely-varying muscle stimulation patterns needed for a 3-D, 8 degree-of-freedom dynamic model to sustain a step. An ankle-foot orthosis was found to be especially useful, as it helped to stabilize the stance leg and simplified the task of controlling the foot during swing. It is believed that the process of simulating natural gait with this model will serve to highlight difficulties to be expected during laboratory and clinical trials.

Computer Simulation↗

PubMed1990

A computer simulation of conduction block: effects produced by actual block versus interphase cancellation.

A reduction in compound muscle action potential (CMAP) amplitude and area following proximal versus distal stimulation is the accepted clinical hallmark of conduction block; however, quantitative criteria for determining conduction block remain ambiguous. In this study, digitized records of individual motor unit action potentials (MUAPs) elicited by incremental stimulation in vivo were arithmetically combined in a computer simulation of CMAP generation. Through simulation of possible phase interaction patterns of individual MUAPs, we have shown that abnormal temporal dispersion alone can produce reductions in CMAP area of up to 50%, values that are commonly thought to represent conduction block. Furthermore, by simulating conduction block without excessive temporal dispersion in defined subpopulations of axons, we have demonstrated the importance of the fastest conducting (largest MUAP) axons in determining CMAP amplitude and area. In conclusion, measurements of CMAP amplitude and area in determining conduction block may be misleading if there is significant abnormal temporal dispersion, and quantitation of the degree of conduction block is difficult without knowledge of which subpopulations of axons are affected.

Action Potentials↗

PubMed1993

Computer simulations of motoneuron firing rate modulation.

1. As a human subject slowly increases the amount of force exerted by a muscle, the discharge rates of low-threshold motor units saturate at a rather low level, whereas higher-threshold units continue to be recruited and undergo increases in their discharge rates. The presently known intrinsic properties of motor units do not produce this "rate limiting." 2. Using computer simulations of a model motoneuron pool, we tested the hypothesis that rate limiting can be accounted for on the basis of the known distributions of synaptic input from different sources. The properties of the simulated motor units and their synaptic inputs were based as closely as possible on the available experimental data. A variety of simulated synaptic input organizations were applied to the pool, and the resulting outputs were compared with the data on rate limiting in human subjects. 3. We found that the data on rate limiting in human subjects greatly constrained the possible organizations of characterized synaptic input systems. Only when the synaptic organization included a gradual "crossover" between two specific types of input systems could the human data be accurately reproduced. Low input/output levels relied on a system organized like the monosynaptic Ia input, which produces greater effective synaptic currents in low- than in high-threshold motor units. Above a sharply defined crossover level, all further increases in output were produced by a system organized like the oligosynaptic rubrospinal input, which generates the opposite pattern.

Computer Simulation↗

PubMed2000

Computer simulations of ultrafiltration profiles for an icodextrin-based peritoneal fluid in CAPD.

BACKGROUND: The three-pore model of peritoneal transport has the ability to predict ultrafiltration (UF) profiles rather accurately, even when high molecular weight (MW) solutes are employed as osmotic agents in continuous ambulatory peritoneal dialysis (CAPD). In the present simulations, we wanted to assess, for various theoretical perturbations, the UF properties of a peritoneal dialysis (PD) solution with an osmotic agent having an average MW of 20 kD and a "number average MW" of 6.2 kD, which is similar to that of icodextrin (ICO). METHODS: For a PD solution containing a completely monodispersed 20 kD MW osmotic agent, the degree of UF modeled is much higher than that reported for ICO. Hence, to model the behavior of ICO, we subdivided the ICO molecules into eight or more different MW size fractions. For simulations using six or eight subfractions, we obtained an excellent fit of simulated to reported UF data. More dispersed solutions produced UF profiles similar to that with eight fractions. RESULTS: A 2.05 L 7.5% ICO PD solution, despite being slightly hypotonic, yielded a UF volume of nearly 600 mL in 12 hours, modeled for patients not previously exposed for ICO. After nine hours, the UF volume exceeded that produced by 3.86% glucose. The UF rate and volumes increased in proportion to (1) the ICO concentration, (2) the peritoneal surface area, and (3) the peritoneal UF coefficient, but was almost insensitive to increases in the instilled fluid volume. Simulated for patients previously exposed to ICO, having steady-state plasma concentrations of ICO degradation products, the predicted UF volume at 12 hours was reduced to approximately 400 mL. CONCLUSION: Employing the three-pore model of peritoneal transport and taking into account the polydispersed nature of ICO, it was possible to accurately computer simulate the UF profiles of ICO in accordance with reported data. The simulations suggest an advantage of using ICO in patients with type I UF failure, where UF with a high-MW osmotic agent will exceed that seen in patients not showing UF failure who are on glucose-based PD solutions.

Absorption↗

PubMed1993

The application of size exclusion chromatography and computer simulation to study the thermodynamic and kinetic parameters for short-lived dissociable protein aggregates.

We describe a method to study the monomer-dimer equilibrium of human growth hormone (hGH) making use of a very short size exclusion high-performance liquid chromatographic column and rapid flow rates. By adjusting the flow rate and thus the retention time on the column, the dissociation of the hGH dimer can be observed. Using computer simulation, both the equilibrium constant for dissociation and the dissociation rate constant can be determined directly, followed by the indirect determination of the association rate constant. This method is potentially useful for determining the thermodynamic the kinetic parameters of aggregation of many protein-protein (homodimers and heterodimers) and protein-ligand systems whose rates of interaction are too rapid to be studied by conventional techniques.

Chromatography, High Pressure Liquid↗

PubMed1993

Computer simulation of alternative sampling strategies to estimate risk of infection from Cryptosporidium.

Estimation of acceptably safe levels of biological contaminants in drinking water requires fitting a mathematical model to infection rates observed in small samples of human subjects. Because of obvious constraints on exposing human subjects to infective conditions, it is not feasible to compare the utilities of alternative sampling strategies and research designs using data from real experiments. Computer simulation methods were used to generate sample data having known probabilities of infection determined by an exponential or log-linear infectivity model. Experimental conditions that were examined included variations in the total available sample size, strategies for allocating subjects among different test concentrations, and methods for fitting a prediction model to the observed data. Results confirmed that data obtained by exposing most subjects to a concentration that produces an infection rate approximating 50% and calculating the sample regression coefficient for the log-linear model as the average infectivity-to-concentration ratio provided the best estimates of safe concentration. Exposing a single subject to each successively higher test level until an initial infection is observed, and exposing all remaining subjects at that level, or an adjacent log-concentration level is a tactic supported by the empirical results.

Animals↗

PubMed2000

A computer simulation of recurrent, excitatory networks of sensory neurons of the gut in guinea-pig.

Intrinsic sensory neurons of the intestine are connected together to form a recurrent network. They interact by slow excitatory post synaptic potentials (EPSPs), which have a complex dependence on the pattern of input. These networks are unstable and unable to give graded responses to sensory input without some form of inhibition, but inhibitory synaptic potentials are rare in this system. Intrinsic sensory neurons have a characteristic after-hyperpolarization (AHP), but this is depressed during slow EPSPs. To test whether AHPs can provide the inhibition necessary for stability, AHPs, slow EPSPs and their interactions were included in a computer simulation of realistic sensory neuron networks. Residual AHPs as small as 1% of control were found to lead to stable networks capable of giving graded responses.

Action Potentials↗

PubMed1986

Computer simulations of nuclear reactions by protons with incident energies of 250, 300 and 500 MeV in a human body.

Computer simulations of nuclear reactions by protons in a human body were carried out at the incident energies of 250, 300 and 500 MeV. About 20% of the incident protons are absorbed by the body with nuclear interactions and the rest of the protons pass through the body at these energies. Radiation of gamma rays from the body and radioactivity of the body were estimated as a function of the time after irradiation with the proton beam.

Body Composition↗

PubMed1991

Model and computer simulations of the motion of DNA molecules during pulse field gel electrophoresis.

A model is presented for the motion of individual molecules of DNA undergoing pulse field gel electrophoresis (PFGE). The molecule is represented by a chain of charged beads connected by entropic springs, and the gel is represented by a segmented tube surrounding the beads. This model differs from earlier reptation/tube models in that the tube is allowed to leak in certain places and the chain can double over and flow out of the side of the tube in kinks. It is found that these kinks often lead to the formation of U shapes, which are a major source of retardation in PFGE. The results of computer simulations using this model are compared with real DNA experimental results for the following cases: steady field motion as seen in fluorescence microscopy, mobility in steady fields, mobility in transverse field alternation gel electrophoresis (TFAGE), mobility in field inversion gel electrophoresis (FIGE), and linear dichroism (LD) of DNA in agarose gels during PFGE. Good agreement between the simulations and the experimental results is obtained.

Chemical Phenomena↗

PubMed1996

Computational simulations of local vascular heparin deposition and distribution.

Local vascular drug delivery systems provide elevated concentrations in target arterial tissues while minimizing systemic side effects; however, definition of their precise pharmacokinetics remains elusive. The standard labeled tracer assays used in experimental vascular pharmacokinetic studies of these systems are limited because they quantify the arterial average drug concentration as opposed to transmural concentration profiles, require many animal experiments to elucidate the time-varying deposition, and track label rather than intact biologically active drug. In this study, computational simulations of drug deposition and distribution in vascular tissues after release from these systems have provided two important insights. First, simulations of arteries that were uniformly loaded with heparin predicted that most of the drug is cleared in < 1 h, illustrating the need for sustained modes of delivery. Second, some of the limitations of labeled tracers can be over come by combining experimental data with simulations that provided high spatial resolution. This enabled us to describe the kinetics of the deposited drug and distinguish soluble from reversibly bound and internalized drug within cells. The latter can help differentiate biologically viable drug from its committed inactive form or metabolites. These points have been illustrated through simulations of a novel endovascular hydrogel heparin-delivery system that has been applied to the porcine coronary artery. The basic models used in these simulations are generalized, and with the appropriate boundary conditions, binding and distribution constants can be used to study the physical interactions between any compound and tissue.

Animals↗

PubMed1997

Biological variation of International Normalized Ratio for prothrombin times, and consequences in monitoring oral anticoagulant therapy: computer simulation of serial measurements with goal-setting for analytical quality.

Oral anticoagulant therapy (OAT) has a well-established efficacy in prophylaxis and treatment of thromboembolic disorders. Because complications are related to intensity of OAT, optimal control of treatment is mandatory. In studies of OAT, as many as 30% of International Normalized Ratio (INR) measurements for prothrombin times fall outside the therapeutic interval. Preanalytical, analytical, and biological variation all contribute to this. Computer simulations of serial INR measurements were performed for various assumed in-treatment setpoints within the therapeutic interval INR 2.0-3.0 and for an "in-treatment within-subject variation" (CV) of 10.1%. Results are presented in difference plots with therapeutic intervals and critical differences. If the in-treatment setpoint is mid-interval (INR = 2.5), only 5% of simulated INR values fall outside the therapeutic interval. Setpoints deviating from the mid-interval and increases in the in-treatment within-subject variation considerably increase the number of observations outside the therapeutic interval and the critical differences. In conclusion, random variation, biological or analytical, and setpoints (targets) deviating from mid-interval explain a substantial number of the INR values outside therapeutic intervals observed in clinical studies. Analytical imprecision should be kept < 5% and analytical bias < +/- 0.2 INR.

Administration, Oral↗

PubMed1982

A theory for the origin of a self-replicating chemical system. II. Computer simulation of the autogen.

In order to better understand the feasibility and limitations of the autogen (White 1980), a computer simulation based on the fluctuating clay environment was used to test whether autocatalytic growth would occur under various conditions. The results suggest that overall accuracies of replication and translation in the range of 90% and 10%, and protoenzyme turnover numbers of 10--120 monomers/protoenzyme/day are adequate for exponential growth. Nucleation of the components of the autogen from random background oligomers would be extremely rapid if oligomers lengths 2--6 were adequately functional, whereas oligomer lengths much greater than 10 are prohibited. The autogen would most likely nucleate and grow to dominance either rapidly (10--100 cycles of roughly 1 day each) or not at all.

Computers↗

PubMed1999

A multi-gate time-of-flight technique for estimation of temperature distribution in heated tissue: theory and computer simulation.

Non-invasive determination of temperature distribution in biological media is important in many heating-related studies, such as thermal treatment. In this paper, we present an in vitro ultrasound technique for estimation of temperature distribution in heated tissue. Our technique consists of two major steps: (1) using multiple time gates to track echo signals scattered from tissue regions at different depths; (2) estimating temperature distribution based on heating-induced changes of arrival times of echo signals scattered from the targeted tissue regions. We use the conventional cross-correlation approach to track echoes. For temperature estimation, we have developed an iterative method that takes into account the influences of thermal expansion and heating-induced change in the speed of sound on the time of flight. We have introduced a concept of thermal sensitivity of the time of flight and used it to derive a theoretical formula that relates the achievable accuracy on the estimation of tissue temperature to seven parameters. The seven parameters are tissue thermal sensitivity of the time of flight, signal-to-noise ratio, bandwidth and center frequency of the signal, degree of signal decorrelation induced by changes in tissue physical properties during tissue heating, and widths and spacing of the time gates. We tested our technique by computer simulation, using a random discrete scatterer model and temperature distribution data acquired in our laser heating experiments on prostate tissue of live dog. Simulation results showed that our technique could accurately estimate the temperature distribution in the heated tissue. Our technique is fast in terms of computation and could be used as a research tool for in vitro real-time monitoring of temperature distribution in tissue under hyperthermal heating.

Acoustics↗

PubMed1987

Effectiveness of a salt transport cascade in the renal medulla: computer simulations.

Based on morphological observations, it has been concluded that the upper parts of the long descending limbs of Henle's loops should be able to secrete salts into the tubular fluid (Kriz, W. Federation Proc. 42:2379-2385, 1983). In the same article, a hypothesis is given depicting how this active transport in concert with certain characteristics of the medullary architecture might produce a transport cascade toward the papillary tip, thus supporting the accumulation of salts in this region. The effectiveness of the proposed mechanism can be judged by a mathematical model only. The computer simulations of the present study demonstrate that this mechanism indeed leads to an increase of the concentrating capability of the renal countercurrent system.

Biological Transport, Active↗

PubMed1988

A computer simulation of hearing aid response and the effects of ear canal size.

The response of a hearing aid is affected by many factors which include the head and outer ear, the microphone, amplifier, and receiver used in the hearing aid, the properties of the ear canal and the eardrum, and acoustic feedback through the vent. This article presents a computer simulation of an in-the-ear (ITE) hearing aid that includes all of the above factors. The simulation predicts the pressure at the eardrum for a frontal free-field sound source. The computer model was then used to determine the effects on the hearing aid response due to variations in the size of the ear canal. The simulation indicates that, for an unvented hearing aid, changes in the size of the ear canal shift the overall sound-pressure level at the eardrum but have only small effects on the shape of the frequency response. The situation is more complicated when a vent is present, however, since changes in the size of the ear canal that cause apparently small perturbations in the acoustic feedback signal may, nonetheless, have large effects on the overall system response.

Computer Simulation↗

PubMed1979

A theoretical approach to precipitin reactions: insight from computer simulation.

The theoretical consequences of different hypotheses of the mechanism of precipitin reactions have been evaluated by means of computer simulation. It has been found that the formation of compositionally different complexes in different antigen/antibody mixtures provides a valid explanation of the zoning phenomenon, but this concept fails to explain the absence of free antigen and of antigen in soluble complexes at the point of maximum percipitation. It is found that the following hypothesis provides an improved qualitative and quantitative explanation of percipitin reactions. In the first stage of the total reaction a series of compositionally different complexes is formed. As the second stage of the total reaction two kinds of processes are proposed. Inherently insoluble complexes precipitate causing the remaining soluble complexes to participate in mutual rearrangements to re-establish a new state of equilibrium in the supernatant. The inherently insoluble complexes, moreover, create a hydrophobic phase, distinct from the supernatant and cause the remaining otherwise soluble complexes to distribute themselves between the two phases according to a partition coefficient. A mathematical apparatus to study the consequences of this hypothesis is presented, and it is demonstrated that the features of precipitin curves can be explained nearly completely this way.

Antigen-Antibody Complex↗

PubMed1998

Topographic mapping in the retinotectal projection by means of complementary ligand and receptor gradients: a computer simulation study.

Based on recent experimental studies of complementary gradients of receptor density (R) on the retinal surface and ligand density (L) on the tectal surface, and mapping of the high point on the receptor gradient to the low point on the ligand and vice versa, the servomechanism model was constructed involving a mechanism for the retinal axon to reach its target automatically sensing a difference between the signal strength (R.L) and the standard value (S). Computer simulations based on the model demonstrated desired two-dimensional topographic mapping of the retinal axons on the tectum, and explained three strange behaviors of the retinal axons that had been observed in stripe assays for retinal axons using stripes composed of tectal membrane fragments: repulsive behaviors of the retinal axons by the ligand substances, uncertainty of the nasal axons whether or not they show regional selectivity between substances of anterior and posterior tecta, and abrupt transition of growth of the axons originating at continuously varied retinal positions on the stripes having graded ligand density. Finally we suggested what is to be improved in stripe assays with the artificial gradient of the tectal membrane fragments.

Animals↗